There are a number of versions of the Telephony Application Programming Interface (TAPI): versions 1.3, 1.4, 2.0, and 2.1. While most of these versions involved changes to the TAPI and Telephony Service Provider Interface (TSPI) documentation sets, there are other implications for each version, namely, architectural differences, operating system variations, redistributables, and telephony service provider (TSP) development issues.